Solid organ transplantation is a life-saving procedure to treat end-stage organ failure. However, transplantation of organs between genetically distinct individuals results in acute graft rejection by the immune system unless patients take immunosuppressive drugs for the rest of their lives. The strength of the anti-transplant immune response, also called the alloresponse, is determined by both genetic and environmental factors. In mouse models of skin and heart transplantation, we have previously found that bacterial infections at the time of transplantation can enhance the alloresponse and promote transplant rejection. This supports the role of microbial infections as environmental factors that can modulate alloresponses, although these are not frequent environmental factors. In contrast, the communities of commensal bacteria that inhabit our body, collectively called the microbiota, share many microbial patterns with infectious bacteria, and are constantly present at barrier surfaces such as the skin and the intestine. Recent evidence indicates that the composition of the microbiota is determined by the immune system and that the microbiota, in turn, drives several effector functions of the immune system. Therefore, we have proposed the hypothesis that the microbiota is an important and omnipresent environmental factor that can enhance alloresponses and promote graft rejection. To address this question, we have used both sterile mice devoid of microbiota and conventional mice treated with broad spectrum antibiotics to reduce microbiota diversity. Our preliminary results show that both sets of mice display prolonged skin graft survival compared with control animals, demonstrating that the microbiota indeed enhances the strength of the alloresponse and accelerates graft rejection. In this application, we propose to investigate the mechanisms by which the microbiota promotes graft rejection, as well as the consequences on the composition of the microbiota of immunosuppression and ongoing alloresponses. Finally, we will attempt to manipulate the microbiota for therapeutic purposes to limit the strength of the alloresponse and prolong graft survival. This research will guide follow up studies in humans to identify the impact of microbiota on alloimmunity and graft outcome. As the composition of the microbiota can be manipulated via anti-microbials, as well as prebiotics (diet), probiotics (beneficial bacteria) and postbiotics (products of bacterial metabolism), this line of research has important possible clinical benefits.